This invention relates generally to techniques for electrically doping semiconducting materials, and more particularly relates to high-concentration doping of semiconducting materials in the fabrication of semiconductor devices.
High-concentration electrical doping of semiconducting materials is becoming increasingly important for enabling advanced-performance electronic and optoelectronic devices. Conventionally, one or more electrical dopants can be added to a semiconducting material by, e.g., in situ incorporation of a dopant during semiconducting material growth, by ion implantation into an existing semiconducting material, or by solid- or vapor-phase diffusion of a dopant into an existing semiconducting material, among a wide range of other doping methods.
Whatever doping technique is employed, the dopant species that is incorporated into a semiconducting material must be electrically activated. That is to say that the dopant species must be positioned at sites in the semiconductor material lattice such that free electrical carriers, i.e., holes or electrons, are contributed to the semiconductor conductivity by the dopant species to alter the conductivity of the semiconducting material in a desired manner.
But the concentration of dopant that is active in a semiconducting material can be much less than the dopant concentration that is actually physically present in the material. Generally, defects in a semiconducting material, e.g., damage that is generated by the doping process itself, can limit the activation of dopants. In general, high-temperature annealing has been shown to both enhance dopant activation and reduce lattice defects. But the temperature that is required for a very high degree of dopant activation by annealing is for many applications too aggressive for integration into advanced semiconductor fabrication sequences with nanometric device features. High temperature annealing processes also can cause a degree of dopant diffusion that is sufficiently high to actually drive the dopant species out of the semiconductor material.
As a result, it is found that for many semiconducting materials, there is some limit of activated dopant concentration beyond which most conventional doping processes fail. For example, ion implantation enables full control of dopant location, including directionality, but for many materials causes severe lattice damage that in general results in a low fraction of activated dopant even when high a high concentration of dopant is physically present. In situ doping during material growth produces relatively minimal lattice damage, but for many materials, high in situ doping can reduce or even halt material growth by, e.g., surface poisoning. The resulting upper limit for in situ doping concentration that can be accommodated during material growth may actually be far below the solid-solubility of the dopant species in the semiconductor material. Solid phase diffusion is limited by the diffusivity characteristics of a given semiconductor material, and vapor-phase diffusion processes typically require a temperature that cannot be tolerated in nano-scale device fabrication with many materials.
For a wide range of important semiconducting materials, high-concentration active doping has therefore remained difficult, and is in general impossible in the context of conventional high-throughput silicon-based fabrication processes and equipment.